Microwave Plasma Chemical Vapor Deposition Research Articles

Epitaxial synthesis of graphene on 4H-SiC by microwave plasma chemical vapor deposition

Epitaxial graphene (EG) on semi-insulating SiC prepared by a thermal decomposition method is the most promising strategy for graphene application in large-scale integrated circuits due to compatibility with current semiconductor processes. In this study, high-quality few-layer graphene (FLG) was epitaxially grown on the semi-insulating on-axis 4H-SiC by microwave plasma chemical vapor deposition (MPCVD). Both sides of the SiC substrate were etched with H2 plasma at ∼1000 °C to promote the nucleation and the growth of graphite before epitaxial growth. The surface morphology and properties of EG on SiC (0001) were measured by energy-dispersive x-ray (EDX), x-ray photoelectron spectrogram (XPS), and atomic force microscope (AFM). The qualities of EG grown on the two surfaces of SiC at various temperatures were checked by Raman spectroscopy. Furthermore, the EG growth was controlled by the effect of Ar plasma in the MPCVD and the few-layer (1–3 layers) high-quality EG films were formed on SiC (000). The room temperature Hall mobility of EGs up to 2790 cm2 V−1 s−1 on SiC(000) was realized.

Heteroepitaxial diamond film deposition on KTaO3 substrates via single-crystal iridium buffer layers

Iridium (Ir) has recently been shown to be a superior buffer layer material for diamond heteroepitaxial growth on different substrates as it possesses chemical inertness and high melting point. This study reports the surface morphologies and microstructures of iridium films deposited on KTaO3 substrates by magnetron sputtering with sputtering powers ranging from 25 W to 100 W at a substrate temperature of ~850 °C. The Ir films were characterized by field-emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). The results showed that the morphologies of Ir films turned out to be more homogeneous and more compact with the increase of sputtering power. In addition, single-crystal Ir films were obtained on all samples as identified by XRD measurements and the crystallographic orientations of the single-crystal Ir films changed from (111) to (100) textures with sputtering power increasing from 25 W to 100 W. The Ir thin film grown at 100 W presented a cube-on-cube-type orientation relationship with KTaO3(100) substrate and a mosaic spread of 0.53°. Then, a thin diamond film (~40-μm-thick) was deposited by microwave plasma chemical vapor deposition (MPCVD) on the Ir buffer layer. FE-SEM, XRD and Raman spectroscopy were used to characterize the heteroepitaxial diamond film. The results indicate that the diamond thin film shows a high degree of epitaxy on the KTaO3(100) substrate with the Ir buffer layer. The mosaicity of the diamond film were 0.42° (tilt) and 0.68° (twist), and the Raman full width at half maximum (FWHM) were typically in the range of 3.6–5.7 cm−1.

Mechanical properties of high-crystalline diamond films grown via laser MPCVD

High-crystalline diamond films were prepared by laser (wavelength: 532 nm) microwave plasma chemical vapor deposition (Laser MPCVD). The effects of laser power density (E) on the microstructure and properties of diamond films were investigated. The laser enhanced the precursor reaction process which reduces the non-diamond components such as graphite and amorphous carbon. At E = 40 W/cm2, more methyl radicals and atomic hydrogen were generated with coupling of laser and plasma, which promoted crystal growth until the grain size reaches the maximum value of 0.72 μm. The phase purity of film reached the highest as the full width at half maximum (FWHM) of Raman peak reached the minimum of FWHMmin = 4.2 cm−1. And hardness (H) and Young's modulus (M) values reached the maximum of Hmax = 91 GPa and Mmax = 721 GPa, respectively. The discovery of the advantages of laser in diamond growth is of great significance for improving the synthesis of many technically crucial materials.

Structural, optical and photoluminescence investigations of nanocrystalline CuO thin films at different microwave powers

Thin films of copper oxide were prepared through two steps: the formation of metallic copper thin films by DC sputtering technique and oxidizing the metallic copper films through microwave plasma chemical vapor deposition technique. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) were used to detect the crystalline phases and the films morphology, respectively. Tenorite CuO phase with interlocking between grains were observed and detected. Optical and photoluminescence properties were investigated by UV–Vis-NIR to detect the optical transparency, optical band gap, and refractive index of the prepared films. The optical transmittance of the studied films was resolved by Swanepoel’s procedure due to the presence of the optical interference in such samples. The optical band gap values were observed to decrease from 2.355 to 1.986 eV as the microwave power increase. A strong UV emission at around 358 nm was observed in all CuO samples. Moreover, weak blue and green emission were also observed in the photoluminescence spectra.

Local initial heteroepitaxial growth of diamond (111) on Ru (0001)/c-sapphire by antenna-edge-type microwave plasma chemical vapor deposition

Heteroepitaxial growth is critical for large-scale synthesis of diamond (111) substrates. In this study, the local initial epitaxial growth of diamond (111) on Ru/c-sapphire was investigated. As the economic viability of ruthenium (Ru) is more than that of iridium (Ir), a 150-nm Ru (0001) thin film was sputter-deposited on an Al2O3 (0001) substrate using a RF/DC magnetron sputtering system. X-ray diffraction analyses of the Ru film revealed the (0001) phase orientation with high crystalline quality. Both bias-enhanced nucleation and initial heteroepitaxial growth of diamond were realized via antenna-edge-type microwave plasma chemical vapor deposition. After 30 min of heteroepitaxial growth, the crystallite (diameter ∼500 nm) with a smooth surface was observed through scanning electron microscopy. Electron backscattering diffraction (EBSD) orientation mapping indicated the presence of the highly oriented diamond (111) crystallite. The epitaxial orientations between diamond (111) and Ru were determined as [111]diamond//[0001]Ru and [112¯]diamond//[101¯0]Ru. The EBSD pole-figure pattern represented the formation of a double positioning defect. This study demonstrated the feasibility of heteroepitaxial growth of diamond (111) on Ru, which provides a more economically viable approach to fabricating large-size diamond substrates.

Bright silicon vacancy centers in diamond/SiC composite films synthesized by a MPCVD method

SiC is always produced forming diamond/SiC composite films owing to the oversaturated doping of Si atoms during the preparation of high emission SiV centers. As a higher refractive index material (n = 2.72), the presence of SiC could possibly lead to inefficient photo-luminescent (PL) collection of SiV centers with total internal reflection. In order to understand such effect, different diamond/SiC films were deposited in a 915 MHz microwave plasma CVD (MPCVD) using the reactive gas of tetramethylsilane (TMS). The microstructure and photoluminescence of SiV centers were systematically investigated. It is found that the addition of TMS gas leads to the formation of cubic β-SiC and the refinement of diamond crystals. At the TMS gas flow of 5 sccm, the concentration of β-SiC is about 10% and the average diamond size is about 98 nm. The film exhibits the optimized PL emission of SiV centers, with the ratio of SiV PL to diamond Raman peak being 21.4. Increasing TMS gas flow leads to the increase of the concentration of β-SiC and the deterioration of PL emission of SiV centers. These results reveal that the introduction of TMS gas at a low flow advances the formation of diamond/SiC film containing high-brightness SiV centers.

Temperature stabilization of WC-Co cutting inserts with feedback to IR pyrometer upon growth of multilayer diamond coatings by microwave plasma chemical vapor deposition

We produced microcrystalline diamond coatings with nanocrystalline top diamond layer on WC-Co cutting inserts in a microwave plasma (2.45 GHz) chemical vapor deposition reactor. The coatings have a high uniformity of the substrate surface owing to elimination of the plasma edge effect by application of a moveable quartz ring above the substrate holder. Computer modeling showed how the positioning of the ring changes the electromagnetic field around the substrates. A new approach was used to stabilize the temperature of the cutting inserts during the coating process, which is based on in situ control of the quartz ring position by proportional-integral-differential (PID) regulation. The coatings have been characterized with microRaman spectroscopy in a mapping mode in cross section of the samples. Depth profiles of diamond Raman peak position and width are demonstrated and the stress in the films is estimated, the minimum stress being determined for the cutting insert apex.

Optical diagnostics of microwave plasma in process of micro/nanocrystalline diamond deposition on hard alloy tools

Diamond coatings on hard alloy tools produced by chemical vapor deposition (CVD) from methane-hydrogen mixtures greatly enhance the functional properties such as wear resistance and quality of the material processing. By varying the gas composition during the diamond film growth it is possible to cyclically change the film structure between microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) to produce MCD/NCD multilayers that improves the mechanical properties of the film and reduces its stress and roughness. As the diamond growth occurs with participation of radicals generated in the plasma thermo-chemical reactions it is important to control in situ the presence of those species and gas temperature Tg. Here we used optical emission spectroscopy (OES) to monitor atomic hydrogen, CN, CH and C2 dimer radicals in course of deposition of MCD/NCD coatings by microwave plasma CVD in CH4-H2 and CH4-N2-H2 mixtures. The gas temperature has been calculated from analysis of rotational components of structure of C2(Δν = 0) emission line. We found that addition of nitrogen (4%) in CH4-H2 plasma, which is needed to form NCD, significantly, by about 500 K reduces Tg (from Tg = 3910 K to Tg = 3330 K). We developed an algorithm tolerable to small shifts of the measured spectral lines which provided fast calculation of Tg, so that the spectrum record and the automatic processing of fine structure of C2(Δν = 0) line totally take the time of about 5 min.

Deposition of diamond films on Si by microwave plasma CVD in varied CH4-H2 mixtures: Reverse nanocrystalline-to-microcrystalline structure transition at very high methane concentrations

Microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films were synthesized on Si substrates by a microwave plasma chemical vapor deposition in methane-hydrogen gas mixtures with a wide range of methane concentrations of 0.5–40%. The samples were investigated with scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. We observed an unusual transition of the produced material structure from MCD to NCD and again to MCD with the increase in CH4 content in the gas. The deposition rate, grain size, and the roughness of the films exhibited extrema in the middle of the methane concentration range ([CH4] = 20%). Further increase of CH4 percentage leads to improvement of the diamond film quality, as confirmed by Raman spectra and SEM images while keeping a high deposition rate. These results may be used for the growth of the wide range of PCD films to be used as multipurpose protective layers, hard coatings for the cutting tools, and the diamond base for composite MCD/NCD structures.

Superhard Boron-Rich Boron Carbide with Controlled Degree of Crystallinity

Superhard boron-rich boron carbide coatings were deposited on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) under controlled conditions, which led to either a disordered or crystalline structure, as measured by X-ray diffraction. The control of either disordered or crystalline structures was achieved solely by the choice of the sample being placed either directly on top of the sample holder or within an inset of the sample holder, respectively. The carbon content in the B-C bonded disordered and crystalline coatings was 6.1 at.% and 4.5 at.%, respectively, as measured by X-ray photoelectron spectroscopy. X-ray diffraction analysis of the crystalline coating provided a good match with a B50C2-type structure in which two carbon atoms replaced boron in the α-tetragonal B52 structure, or in which the carbon atoms occupied different interstitial sites. Density functional theory predictions were used to evaluate the dynamical stability of the potential B50C2 structural forms and were consistent with the measurements. The measured nanoindentation hardness of the coatings was as high as 64 GPa, well above the 40 GPa threshold for superhardness.

Effects of Substrate Bias on the Diamond Nanostructure by Reactive Ion Etching

Dense and uniform diamond nanocone arrays on titanium substrate were constructed by using bias-assisted reactive ion etching (RIE) of diamond films in a microwave plasma chemical vapor deposition (MPCVD) system. A hydrogen/argon mixture was employed as work gas with ratio of 3/1. The formation of nanocone structure was generated the lengthways physical bombardment/sputtering by argon ions, and selective chemical reaction of graphite and disordered carbons by hydrogen atoms and ions. The structure, size, and density depended on the substrate bias. The surface morphology of diamond film and nanostructures were characterized by field emission scanning electron microscope (FESME), and the composition of diamond film and nanostructures were characterized by Raman spectra. The Stopping and Range of Trons in Matter (SRIM) software was used to simulate the bombardment effect of the diamond film on different Ar+ ion incident angles during the etching process. The experimental results showed substrate bias at −180 V and −210 V was beneficial to the formation of high density, small size and sharp nanocones, meanwhile larger bias contributed to the formation of large size nanocones. Besides, as-prepared nanocones still maintained significant diamond phase.

Sulfur regulation of boron doping and growth behavior for high-quality diamond in microwave plasma chemical vapor deposition

In this work, sulfur addition has been employed on the boron-doped diamond growth process, and a significant regulation of the boron doping and the growth behavior has been realized by microwave plasma chemical vapor deposition. It is interesting to find that the sulfur incorporation will lead to an accordant evolution on the boron doping efficiency, hole mobility and concentration, crystal quality, surface morphology, and growth rate. In the presence of sulfur with appropriate dosage, for a boron-to-carbon ratio of only 2.5 ppm in gas phase during growth, a very high doping concentration of 1.2 × 1019 at/cm3 has been achieved, indicative of a very efficient boron doping. Besides, the hole mobility of the sample is 853 cm2/V s at 300 K, which is better than the state of the art for p-type doping in diamond. The regulation mechanism of the sulfur addition will be discussed from the point of view of sulfur-induced plasma change and possible B–S complex formation. This study may provide an effective way for high-quality p-type conductive diamond layer growth and further for the potential diamond-based opto-electronic device applications.

Relationship between Emission Spectrum and B Content in B Doped Diamond Synthesis using Mode Conversion Type Microwave Plasma CVD

Relationship between Emission Spectrum and B Content in B Doped Diamond Synthesis using Mode Conversion Type Microwave Plasma CVD

Synthesis of Multiwalled Carbon Nanotubes on Stainless Steel by Atmospheric Pressure Microwave Plasma Chemical Vapor Deposition

In this paper, we synthesize carbon nanotubes (CNTs) by using atmospheric pressure microwave plasma chemical vapor deposition (AMPCVD). In AMPCVD, a coaxial plasma generator provides 200 W 2.45 GHz microwave plasma at atmospheric pressure to decompose the precursor. A high-temperature tube furnace provides a suitable growth temperature for the deposition of CNTs. Optical fiber spectroscopy was used to measure the compositions of the argon–ethanol–hydrogen plasma. A comparative experiment of ethanol precursor decomposition, with and without plasma, was carried out to measure the role of the microwave plasma, showing that the 200 W microwave plasma can decompose 99% of ethanol precursor at any furnace temperature. CNTs were prepared on a stainless steel substrate by using the technology to decompose ethanol with the plasma power of 200 W at the temperatures of 500, 600, 700, and 800 °C; CNT growth increases with the increase in temperature. Prepared CNTs, analyzed by SEM and HRTEM, were shown to be multiwalled and tangled with each other. The measurement of XPS and Raman spectroscopy indicates that many oxygenated functional groups have attached to the surface of the CNTs.

Photoluminescence Properties of GaN Nanowires Grown in a Gradient-Plasma Environment

Herein, we report the plasma effect on the surface defects of GaN nanowires (NWs). A gradient plasma growth system by microwave plasma chemical vapor deposition is demonstrated for NWs growth. The structure and surface defects of NWs are affected by the plasma distribution. The PL results show that emission peaks at ∼355, ∼374, and ∼389 nm arising from near band edge emission are observed from the GaN NWs under different plasma growth environment. The relative emission area (Erelative) of the specific emission peaks to total emission area (Etotal) from the different growth regions also changes. The reason for variant ratio Erelative/Etotal is related to the surface defects including structural defects and oxygen incorporation according to the results of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The analysis of PL properties provided a strong evidence for formation of distinct surface defects. The emission mechanism of GaN NWs grown in a high-density plasma environment enriches our understanding of the plasma effects on the surface defects and optical property, and the knowledge is important to the development of nano-optoelectronic devices.

Controlled Doping of GeV and SnV Color Centers in Diamond Using Chemical Vapor Deposition.

Group IV color centers in diamond (Si, Ge, Sn, and Pb) have recently emerged as promising candidates for realization of scalable quantum photonics. However, their synthesis in nanoscale diamond is still in its infancy. In this work we demonstrate controlled synthesis of selected group IV defects (Ge and Sn) into nanodiamonds and nanoscale single crystal diamond membranes by microwave plasma chemical vapor deposition. We take advantage of inorganic salts to prepare the chemical precursors that contain the required ions that are then incorporated into the growing diamond. Photoluminescence measurements confirm that the selected group IV emitters are present in the diamond without degrading its structural quality. Our results are important to expand the versatile synthesis of color centers in diamond.

A review of graphene synthesisatlow temperatures by CVD methods

Chemical vapor deposition (CVD) is the most effective method for the synthesis of large-scale and high-quality graphene. However, the growth temperature of graphene is high, about 1000 °C, using conventional CVD, meaning that it is expensiveand thus limits the use of the material. The synthesis of CVD graphene at low temperatures(<600°C) is therefore the focus of many researchers. Recent research on the production of CVD graphene at low temperaturesis reviewed. Comprehensive comparison, analysis and discussion of quality, number of layers, domain size and the uses of graphenesynthesized at low temperatures using different precursors (gas, liquid and solids) and substrates (metals, metal alloys and dielectric materials) are given for different CVD methods (atmospheric pressure CVD, plasma enhanced CVD, catalyst-enhanced CVD, surface wave plasma CVD, microwave plasma CVD, radio frequency plasma enhanced CVD and electron cyclotron resonance CVD). The future prospects and challenges of preparing graphene at low temperatures are discussed.

Engineering high-defect densities across vertically-aligned graphene nanosheets to induce photocatalytic reactivity

The fabrication of graphene nanostructures, with a variety of morphologies and densities of defective sites, can be a promising tool to tune their characteristics towards photocatalytic applications, without the need for external dopants. In this study, the impact of morphological properties in terms of the orientation and defect concentrations of graphene nanostructures is demonstrated to support the development of active photocatalytic sites across graphitic structures. Vertically-aligned graphene nanosheets were grown across carbon fibres via electron cyclotron resonance microwave plasma chemical vapour deposition, to yield a range of different wall densities and edge functionalities. The variation of growth conditions was correlated to the photocatalytic activity for the degradation of methylene blue dye under ultra-violet and visible light. The chemical state of oxygen content hybridized with nanosheets was studied by X-ray photoelectron spectroscopy and correlated to the growth conditions and photocatalytic performance. The fastest degradation rate of dye was found on the graphene samples which were grown at 800 °C for 240 min, with a kinetic constant of 46.6 × 10−4 min−1. Such performance has not been observed to date for any graphitic materials and is shown to be on the same order of performance as the conventional photocatalytic materials.

Cross‐Sectional Investigation of Multihigh‐Energy Proton‐Implanted Single Crystalline Diamond

Comprehending the implantation‐related characteristic quantities of multihigh‐energy proton irradiation of diamond is of significant technological interest for sensing applications, fabrication of 3D photonic crystals, and color centers (e.g., NV− center)‐based quantum computing. Herein, proton‐projected ranges and defect distributions in the (010) plane (Y‐normal plane) of microwave plasma chemical vapor deposition (MPCVD)‐grown diamond irradiated with 0.5–2.0 MeV proton (H+ or molecular ions) in a nonchanneling (near [001], Z‐axis) direction, are experimentally measured via cross‐sectional optical microscopy and high‐resolution confocal Raman spectroscopy mapping. It is found that the projected ranges of the experimental findings and the simulation results indicate good consistencies, whereas there is considerable disagreement between them in regards to longitudinal straggling and lateral straggling. Based on the comparisons and analyses, a clear view of protons implanted in single crystalline diamond in the high‐energy range theoretically and experimentally is obtained.

Cutting performance of cubic boron nitride-coated tools in dry turning of hardened ductile iron

A novel cubic boron nitride (cBN)-coated silicon nitride(Si3N4) cutting tool was fabricated via the diamond interlayers by electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD) technique. The cutting performance of cBN-coated tools was evaluated by dry turning of hardened ductile iron in comparison with the commercially available titanium aluminum nitride (TiAlN)-coated tools. The effects of cutting parameters on the cutting force and cutting temperature were studied. The results showed that the cutting forces increased with the increase of depth of cut but decreased with the increase of cutting speed, while the cutting temperature increased with the increase of cutting speed. Compared to the TiAlN-coated tools under identical turning conditions, the cBN-coated tools had lower cutting force and cutting temperature, and smaller tool wear. The cBN-coated tools delivered excellent cutting performance and prolonged tool life. Furthermore, the wear patterns and wear mechanisms of cBN-coated tools were investigated. It was found that the dominant wear patterns were crater wear on rake face and flank wear land, and the wear mechanisms were demonstrated to be abrasion wear and diffusion wear. The cutting experiments verified that the cBN-coated tools had attractive potentials in the applications for dry turning of refractory metals and hardened ferrous materials.